The present invention relates to a brake system destined for automotive vehicles comprising a dual-circuit hydraulic, pedal-actuated braking pressure generator which is connected to an auxiliary energy source and the two brake circuits of which communicate each with a front wheel and a rear wheel, comprising braking pressure modulators contained in the pressure fluid lines leading to the rear wheels, and comprising pressure fluid lines leading from the rear-wheel brakes to a pressure supply reservoir and containing separating valves adapted to switch to the open position. The brake system further comprising switchable valves for reducing the boosting effect by decreasing the auxiliary energy transmitted onto the braking pressure generator and further comprising sensors and electric circuit configurations for the determination and evaluation of the wheel rotational behavior as well as for the generation of control signals which serve to influence the distribution of braking pressure onto the front wheels and the rear wheels and the braking pressure variation upon the occurrence of tendencies to lock.
A number of brake systems with slip control are known. Sensors, such as inductive transducers, serve in such systems to directly or indirectly measure the rotational behavior of the wheels, and braking pressure variation will be taken influence on, if the test values or, respectively, the time variations allow to detect an imminent locked condition. By means of modulators, which may be designed as controllable valves also, the tendency to lock will be counteracted by decreasing, maintaining constant and, in case of need, re-increasing the braking pressure at the wheel concerned or at jointly controlled wheel groups according to predetermined relations, with a view to controlling the wheel brake slip to adopt a value favorable for slowing-down, driving stability and steerability of the vehicle.
Since the adherence of the wheels on the road as well as the forces caused at the wheel during braking are dependent on various parameters, the most important ones whereof vary within very wide limits, such brake slip control apparatuses are of costly design. It is therefore the main object to be achieved in the development of brake slip control apparatuses to simplify these apparatuses and to reduce the manufacturing effort entailed, while no losses in control accuracy or only minor ones are encountered.
A brake system incorporating an auxiliary energy source is described already in German printed and published patent applications DE-OS 30 40 561 and DE-OS 30 40 562. The braking pressure generator in this system comprises a braking pressure control valve which is controlled by means of the brake pedal and to which a brake circuit is connected. Via a single-type or tandem-type master cylinder which is unitized with the braking pressure control valve and which is likewise acted upon by the brake pedal force boosted by the auxiliary energy, one or two further brake circuit(s) is/are connected. For the purpose of brake slip control, electromagnetic switching valves inserted in the hydraulic brake circuits are modulated, which valves are able to shut off the supply of the hydraulic medium to the wheel brake and thereby preclude further braking pressure increase. The braking pressure will be decreased to attain the pressure level desired by opening of an outlet towards the pressure supply reservoir. Further switching valves permit dynamic pressure out of the auxiliary energy source to enter into the working chambers of the master cylinder, as a result whereof the discharge of hydraulic medium into the supply reservoir will be compensated for, and exhaustion of the working chambers due to repeated pressure decrease will be prevented. Such systems necessitate rather high expenditure of hydraulic means.
Systems with open-center boosters are known (German printed and published patent applications DE-OS 30 40 548 and DE-OS 30 40 540), wherein the pressure fluid conduit of the pump feeding the auxiliary pressure contains a throttle valve which does not close until brake actuation, which thereby initiates pressure build-up in the auxiliary energy source and ensures brake force boosting. Apart therefrom, the design of the hydraulic unit consisting again of a brake valve and a master cylinder, as well as the arrangement of the solenoid valves which will be actuated when a control action takes is similar to the system described hereinabove.
Another problem arising in the dimensioning of brake systems is the adaptation of brake force distribution to the static and dynamic loads on the vehicle axles. Conventional brake force distributors are confined to an invariably set, pressure-responsive control. Load-responsive or deacceleration-responsive brake force control units are likewise known in various designs. All these devices permit to attain but a relatively coarse adaptation to the actual axle load distribution.
Further, a brake force distributor is known already wherein, when the vehicle is at a standstill, the static axle load distribution is measured by sensors and is delivered into a microcomputer which, in consideration of these test values and of the measured braking pressure, will modulate the brake force distribution onto the front axle and the rear axle according to a memorized mathematical relation (European patent application EP-A1 062246). Likewise such a power brake booster bears the disadvantage that only a calculated frictional value, but not the actual one prevailing at the moment the brake action takes place, is decisive for the brake force distribution. Therefore, for reliably avoiding the dangerous overbraking of the rear axle, the brake system is required to be dimensioned such that, in the majority of cases, the effect the rear axle takes in braking is less than would be possible physically. Moreover, it has to be considered that in practice the actual values most frequently differ widely from the calculated present nominal values.
The brake characteristic values assumed as being constant when dimensioning the brake force distribution and in respect of the memorized mathematical relation are subject to considerable changes due to manufacturing tolerances, variations in temperature, wear etc. For this reason, it has been proposed to ascertain the brake slip at front and rear wheels by means of wheel and vehicle sensors as well as by logic combining circuits, and to control the brake slip at the rear axle in dependence upon the brake slip of the front axle such as to ensure that with each braking action there occurs at the rear wheels approximately the same coefficient of friction as at the front wheels (German patent application P 33 01 948.7).
However, a shortcoming of such a brake force distributor is that in the case of overbraking (i.e., in the event of excessive brake application) the front wheels are the first to lock and only in the presence of higher pressure will be rear wheels follow. This is important because the locking of the rear wheels ensues great risk of skidding, while the loss of steerability due to locking wheels is less severe compared thereto. When the brake is depressed excessively, none of the brake power boosters described prevent locking of both the front wheels and the rear wheels and therefore the vehicle is deprived of steerability and driving stability as a consequence thereof.
Therefore, a brake system system has been proposed which, on any braking action, both controls brake force distribution onto the front axle and the rear axle and prevents locking of the wheels. The hydraulic units and braking pressure modulators required for this aim will be employed for the control of the brake force distribution and also for brake slip control so that the manufacturing effort made for the brake system becomes relatively small as a whole. The brake system mentioned hereinabove and described in German patent application P 33 23 402.7 distinguishes by being particularly straightforward, while it is nevertheless able to distribute the brake force optimally onto the front wheels and the rear wheels with every braking action, what has as a result uniform adherence ability at the front and the rear axle, and in addition to prevent locking of the wheels, whereby driving stability and steerability of the vehicle are maintained even during panic stops on icy roads, etc.
However, in the event of a controlled reduction of the braking pressure at the front axle, the pressure variation at the front-wheel brakes is not entirely uniform in the brake system according to the main patent, wherein the braking pressure generator is composed of an open-center booster combined with a master cylinder and having its throttle valve direclty communicating with one of the two brake circuits. The reason therefore is to be seen in that, even when deactivating or, respectively, shortcircuiting the auxiliary energy source, the pedal force continues to act in the master cylinder circuit, but not in the circuit connected to the throttle valve. In consequence thereof, a specific residual pressure is left in the master cylinder circuit.
It is therefore an object of the present invention to improve the braking pressure variation at the front-wheel brakes, in particular during decrease of the braking pressure, or, respectively, to bring about a still closer approximation to the ideal pressure variation, without necessitating additional components at all or only a few thereof. Moreover, the behavior upon pressure failure in a front-wheel brake circuit is wished to be controllable in the desired fashion--according to the respective embodiment and the special requirements existing.